Telephone line integrity monitoring system

Abstract

A system for monitoring for integrity of a telephone line connected between a central monitoring station and a subscriber's equipment which comprises a first module for initiating transmission of an identification code from the subscriber's equipment along the telephone line to be monitored to the central monitoring station during an incoming ringing signal; a recognition module response to said transmitted identification code for recognizing the identification code; and a second module for dropping the line after said transmitted identification codes has been recognized and before the line has been picked up.

We claim: 
     
       1. An alarm system comprising: telephone line integrity monitoring means for monitoring the integrity of a telephone line connection between a first piece of equipment and a subscriber's equipment including means for initiating transmission of an identification code from the subscriber's equipment along the telephone line to be monitored to the first piece of equipment during an incoming ringing signal;   recognition means responsive to the transmitted identification code for recognizing the identification code; and   means for dropping the line after the transmitted identification code has been transmitted and before the line has been picked up.     
     
     
       2. A system according to claim 1, wherein the first piece of equipment is a central monitoring station. 
     
     
       3. A system according to claim 1, wherein the first piece of equipment is a further subscriber's equipment. 
     
     
       4. A system according to any one of the proceding claims, in which the subscriber's equipment comprises an intruder detection alarm. 
     
     
       5. A system according to claim 1, in which the transmission means comprises a digital communicator. 
     
     
       6. A system according to claim 1, in which the identification code is transmitted along the telephone line to be monitored between the first and second power ringing signals. 
     
     
       7. A system according to claim 1, in which the recognition means comprises a digital receiver. 
     
     
       8. A system according to claim 1, in which the subscriber's equipment is connected to the first piece of equipment via the Public Switched Telephone Network (PSTN). 
     
     
       9. A system according to claim 5, in which the means for dropping the line after the identification code has been transmitted is associated with the subscriber's digital communicator. 
     
     
       10. A system according to claim 1, in which both the first piece of equipment and the subscriber's equipment are provided with digital communicators having associated therewith line dropping means. 
     
     
       11. A system according to claim 9, in which the line dropping means comprises a tone detection circuit which can detect the ringing tone at the receiver, and a timer which drops the line at a predetermined interval after the ringing tone has been detected. 
     
     
       12. A system according to claim 10, in which the digital communicators are provided with an answer delay such that in normal, non-alarm conditions, the line drop means is activated by the timer before the line is picked up. 
     
     
       13. A system according to claim 1, in which the first piece of equipment and the subscriber's equipment are each provided with a timer or timers which are preset such that the first piece of equipment expects to receive a call from the transmitting means at predetermined intervals. 
     
     
       14. A system according to claim 1, in which the first piece of equipment is provided with a timer such that the first piece of equipment prompts each subscriber's transmitting means at predetermined intervals, to acknowledge that it is still connected and operational. 
     
     
       15. A system according to claim 1, in which the first piece of equipment is connected to equipment at a plurality of subscribers. 
     
     
       16. A method of monitoring the integrity of a telephone line connection between a first piece of equipment and a subscriber's equipment which comprises: initiating transmission of an identification code by means of the subscriber's equipment along the telephone line to be monitored to the first piece of equipment during an incoming ringing signal;   recognising the identification code by means of recognition means at the first piece of equipment; and   dropping the line after said transmitted identification code has been recognised and before the line has been picked up.   
     
     
       17. A method according to claim 16, in which the first piece of equipment is provided with a timer or timers set for each subscriber, and expects to receive a call from each of the subscribers at predetermined time intervals as established by the timer or timers. 
     
     
       18. A method according to claim 17, in which, after each subscriber's transmitting means initiates the transmission of its identification code to the first piece of equipment, the recognition means decodes the caller identification code and resets the first piece of equipment's timer for that subscriber and in which, if the subscriber's equipment has not changed state since it last reported its status, the subscriber's equipment drops the line after the subscriber's identification code has been transmitted and recognised. 
     
     
       19. A method according to any of claims 16 to 18, in which, if the first piece of equipment does not receive a call from a monitored subscriber's equipment in the pre-determined time period, the central station calls that subscriber's equipment, and, when the central station has initiated the transmission of its identification code to the subscriber's equipment it drops the line; and when the subscriber's equipment recognises the identification code of the first piece of equipment, it transmits the subscriber's identification code to the first piece of equipment and drops the line, thus acknowledging the previous call from the first piece of equipment. 
     
     
       20. A method according to claim 16, in which the first piece of equipment prompts each subscriber's equipment, at pre-determined intervals, to acknowledge that it is still connected and operational. 
     
     
       21. A method according to claim 20, in which the first piece of equipment transits a call to each subscriber in turn, at a period dependent upon the assessed potential risk, and after its identification code has been transmitted drops the line; recognition means associated with the subscriber's equipment recognises the identification code from the first piece of equipment; and the subscriber's equipment calls back the first piece of equipment to acknowledge the communication, dropping the line after its identification code has been transmitted. 
     
     
       22. A method according to claim 16, in which, if the first piece of equipment does not receive an appropriate call or acknowledgment from a subscriber's equipment, or if the subscriber's equipment continues to ring the first piece of equipment after its identification code has been transmitted and recognized, the first piece of equipment reports that an abnormal event has been detected.